The present invention relates to optical devices making it possible to produce in a non-linear propagation material a radiation, whose wavelength is a sub-multiple of a wavelength of another radiation applied to said material. It more specifically relates to integrated structures in which the two radiations interact in non-linear manner in phase matching within a waveguide obtained by the diffusion of metal ions from the surface of a substrate.
It is known to produce integrated waveguides partly making it possible to convert a fundamental wave into a harmonic wave by the diffusion of titanium into lithium niobate. However, the conversion efficiency of such a structure is low. Thus, the confinement and considerable interaction length of the waves are not the only factors to be taken into consideration when seeking an adequate conversion efficiency. The necessity of satisfying the phase matching condition implies a choice of propagation modes for which there is a low overlap integral value.
In order to solve the problem posed by the choice of modes able to offer a suitable overlap integral value it has been proposed to cover a waveguide with zinc zulphide by means of a titanium dioxide layer. However, this method cannot be used on guides obtained by the diffusion of titanium into lithium niobate because the modes providing the phase matching have a limited overlap integral. Thus, phase matching can be obtained between the zero mode of the fundamental wave and higher order modes of the harmonic wave. If the effective indices of the two first harmonic modes are higher than the effective index of the fundamental mode the addition of a passive layer to the guidance structure leads to an increase in the effective indices, and as a result the first phase matching occurs for a higher rank harmonic mode than that which can be used in the absence of said passive layer.